This invention is related specifically to piezoresitive sensors for microelectromechanical systems (MEMS), and specifically to organic semiconductor piezoresistive sensors.
Microelectromechanical systems (MEMS) include devices with features having a size less than 100 microns in at least one dimension, and preferably in two or three dimensions. Preferably, these features comprise movable features or elements, such as cantilevers, diaphragms, clamped beams, wires, etc. Microelectromechanical systems include, but are not limited to, scanning probe microscopes (SPM), such as atomic force microscopes (AFM), force and pressure sensors, flow sensors, chemical and biological sensors, and inertial sensors, such as accelerometers and motion transducers. For example, chemical and biological sensors may comprise one or more cantilevers having a surface coated with a material which selectively binds to a chemical or biological analyte (i.e., gas or liquid analyte containing or consisting of the chemical or biological species of interest).
Piezoresitive displacement detection techniques are attractive in MEMS because they are able to be fully integrated and are easy to use. Most of these applications use p-type doped silicon layer as the piezoresistive sensing element.
Silicon has traditionally been used as a piezoresistive strain sensor due to its high piezoresistance coefficients and thus high sensitivity. However, silicon is very stiff, with a Young's modulus of 1011 Pa which reduces sensitivity. Piezoresistance in gold wires integrated into Su8 structures have been utilized in order to take advantage of the lower Young's moduli in gold and Su8. However, the piezoresistance coefficients in gold are small compared to those of silicon.
One application of sensors is for the coupling to a single cell for measuring forces exerted by the cell on its surroundings, thereby probing the structural state of the cell cytoskeleton. It is known that the structural state of the cytoskeleton is inseparably linked to the functional status of the cell. Early measurements probing the cytoskeleton have been made with micromanipulated microbeads, micropipettes and microfabricated post-array-detectors. These measurements are limited in spatial resolution (down to approximately 10 microns), force resolution (down to approximately 10 nanonewtons) and time resolution (only single shot measurements were possible).
Traction force microscopy, a more recent technique, observes the displacement of fluorescent beads suspended in a polyacrylamide membrane while a cell migrates across the membrane. Traction force microscopy achieves improved spatial resolution (down to approximately 3-4 microns), force resolution (down to approximately 500 piconewtons) and time resolution (down to approximately 40 seconds). However, the system requires unobstructed optical access to the membrane which prevents integration of force actuators and microfluidics.